106 FOSSIL REPTILTA OF THE 



All are enveloped in one sheath of smooth integument, which is continued beyond the 

 framework to an obtusely pointed end, and likewise extends some way beyond the 

 hinder or ulnar border of the bones, where it seems to have been supported by numerous 

 fine cartilaginous rays unless the appearance be interpreted as due to parallel bifurcate 

 wrinkles of the skin (PI. XX VIII, fig, 5). 1 



The ilium (PI. XXX, fig. 5, G2) is an oblong, subcompressed bone, slightly enlarging 

 at the acetabular end to form the articular surfaces for pubis, ischium, and femur ; some- 

 times, also, more slightly expanding to the opposite free flattened end. This part, if 

 attached to any of the vertebra, would be so by ligamentous or soluble tissue. The length 

 of the ilium does not exceed, usually, that of three contiguous centrums. 



The ischium (ib. 63), of minor length, is thickened at the acetabular end, which shows 

 facets for the ilium, the pubis, and its share in the shallow styloid cavity for ligamentous 

 attachment of the head of the femur. The body of the ischium expands, more or less in 

 different species, to its free non-articular border, anterior to which it unites synchondro- 

 sally with the pubis. This (ib., 64) is usually a somewhat longer bone; it also expands 

 to join the ischium at its lower end ; the opposed borders of the two bones are concave, 

 and inclose a long and large ' obturator ' vacuity. 



Modifications of these pelvic elements are shown in Ichthyosaurus communis and 

 Ic/i. lati/rons, and will be noticed in some other species. 



The femur (PI. XXXI, fig. 1, Go) resembles the humerus, but is smaller and usually 

 rather less broad. Its distal end supports two cnemial bones, repeating the shape and 

 relative proportions of the antibrachial ones. To these succeed three tarsal ossicles, Avhich 

 support four series of digital ossicles. But in Ich. communis the anterior or tibial series 

 soon bifurcates, and supplementary ossicles are applied to those of the fibular or hinder 

 digit. The shape of the ventral or pelvic fin thus supported is similar to, but relatively 

 less broad (in Ich. communis) than, the pectoral one. 



The rare illustration given in Plate XXVIII, fig. 5, is of a pelvic fin. 



d. Osteological Summary. — At the risk of some repetition the following remarks, 

 the result of such comparisons as I have had time or means of extending over the cold- 

 blooded air-breathers, may not be unacceptable. The comparison with existing Repiilia 

 is almost restricted to the Lacertian and Crocodilian modifications. 



In Crocodiles the exoccipitals leave an intervening tract of the basioccipital, which 

 thus takes its share in support of the ' medulla oblongata'; in Lizards the exoccipitals meet 

 upon the basilar element, as in Ichthyosaurus, but the extinct Reptile differs from both 

 in the exclusive formation, by the basioccipital, of the condyle articulating with the 

 atlas. Such joint in Fishes is afforded by the basioccipital only, but the articular surface, 

 as a rule, is concave instead of convex. The superoccipital is excluded from the foramen 



1 ' Transactions of the Geological Society,' 2nd series, vol. vi, p. 199, pi. xx. 



